The recent trend in the technology of insulated wires, especially coaxial cables for high-frequency signal transmission, is toward the use of cellular insulating coating having a high expansion ratio for the lowering of dielectric constant and the .delta., which leads to a reduction of crosstalk and attenuation, an improvement of image and sound, and a decreased number of repeaters required.
The production of wires insulated with a cellular material having a high expansion ratio resorts usually to the chemical blowing process or gas blowing process. The chemical blowing process consists of compounding a resin component with a chemical blowing agent at a temperature under the decomposition point of the chemical blowing agent, extruding the resulting compound at a temperature above the decomposition point of the chemical blowing agent, thereby coating a conductor, allowing the coating layer to expand in the air, and cooling and solidifying the expanded coating layer. The chemical blowing process is simpler in operation and more economical in equipment than the gas blowing process. Therefore, it has gained general acceptance although it is inferior to the gas blowing process as far as the expansion ratio is concerned.
A major resin used in the chemical blowing process has been high-pressure low-density polyethylene (abbreviated as HP-LDPE hereinafter). This is because HP-LDPE has no polar groups in the molecule and, hence, has a low dielectric constant, e.g. tan .delta., and is suitable for use as a cellular insulation for coaxial cables. In addition, HP-LDPE has a melting point of typically about 107.degree. C. and, hence, is capable of processing at a lower temperature than high-density polyethylene having a high melting point (typically about 133.degree. C.). Moreover, HP-LDPE is easy to make into a highly-expanded foam because it has a higher melt tension than high-density polyethylene.
There has recently arisen a demand for the improvement of coaxial cables in characteristic properties such as dielectric constant, e.g. tan .delta., for the reduction of crosstalk and attenuation. To meet this demand, it is necessary to increase the expansion ratio of the insulation coating up to 60% and above, preferably 70% or more.
However, difficulties are involved in the production of a highly-expanded (above 60%) foam from HP-LDPE. That is, the foam has such a thin cell membrane and, hence, has such a low mechanical strength that the resulting coaxial cable has poor mechanical properties such that it collapses or is broken easily during handling.
To address this problem an attempt has been made to incorporate HP-LDPE with high-density polyethylene, which is superior in mechanical strength to HP-LDPE. For example, Japanese Patent Laid-Open No. 54:44787/1979 discloses a process for producing a highly-expanded foam with high mechanical strength by gas blowing a resin composition composed of 100 parts by weight of high-density polyethylene and 20 to 50 parts by weight of low-density polyethylene. Unfortunately, this process is not applicable as such to the chemical blowing method.
Japanese Patent Laid-Open Publication No. 58:174423/1983 discloses a mixture composed of a low-density polyethylene having a melt index of 1 to 7 grams/10 minutes and a high density polyethylene. The invention is characterized by the use of two chemical blowing agents and not by the combination of two polyethylenes. In the examples, only low density polyethylene is employed and not a mixture of a low density polyethylene and a high density polyethylene.
The production of a highly-expanded foam (having an expansion ratio greater than 60%) from HP-LDPE incorporated with high-density polyethylene by the chemical blowing process requires that the resin should have a sufficiently high melt tension at its blowing temperature. For this reason, the blowing temperature is restricted in the range of 130.degree. C. to 160.degree. C. and, hence, the decomposition temperature of the chemical blowing agent is also restricted in the range of 130.degree. C. to 160.degree. C. If a chemical blowing agent having a decomposition temperature of 130.degree. C. to 160.degree. C. is to be uniformly dispersed into HP-LDPE incorporated with high-density polyethylene, it is necessary that mixing should be performed at a temperature higher than the melting point (typically about 133.degree. C.) of high-density polyethylene. Such mixing, however, induces the decomposition and foaming of the chemical blowing agent, making its impossible to produce an expandable resin composition.
For the reasons mentioned above, it was difficult to produce a highly-expanded polyethylene (having an expansion ratio greater than 60%) from HP-LDPE incorporated with high density polyethylene.
The inventors found it possible to solve the above described problems by mixing low density polyethylene and high density polyethylene by a special method, and filed Japanese Patent Application Laid-Open Publication No. 4-76028/1992.
Recently, telecommunication instruments, and image or picture processing instruments, such as telephones, radios, facsimile machines, copy machines, computers, word processors, and television, have become remarkably smaller in size, and their parts have become smaller in size and have been clustered into compact configurations in high densities. To cope with this tendency, smaller-diameter electric wires insulated with highly-expanded cellular polyethylene have been increasingly used in the above mentioned instruments as compared with the conventional larger diameter wires insulated with highly-expanded cellular polyethylene, such as coaxial cables.
For the production of such larger-diameter coaxial cable, for example, the production has been conducted with no trouble by the method, such as our invention of the above-mentioned Japanese Patent Laid-Open Publication No. 4-76028, in which an expandable resin composition is applied to the larger-diameter wires. However, when the expandable resin composition disclosed in our invention of Japanese Patent -Laid-Open Publication No. 4-76028 was applied to a smaller diameter wire, the resultant cellular polyethylene did not attain an expansion ratio as high as above 60% which was required for use. This has been a problem which has been needed to be solved.